1. Field of the Invention
The present invention relates to a method of preparing an oxide superconducting material, and more particularly, it relates to a method of preparing an oxide superconducting material which is provided as a film formed on a substrate.
2. Description of the Background Art
In recent years, oxide superconducting materials having high critical temperatures have been successively found and extensive study has been made in order to put such materials into practice for wires and the like. In particular, various methods have been studied for working such oxide superconducting materials into wires.
An oxide superconducting wire is typically prepared by a silver sheath method of filling a silver pipe with raw material powder and properly carrying out wire drawing, rolling and heat treatment steps for obtaining a tape-type wire. This method is effectively applied to preparation of a wire, since the material can be easily elongated and a relatively high critical current density can be attained by mechanical orientation in rolling.
However, the wire obtained by the silver sheath method merely exhibits a critical current density of a little less than 50,000 A/cm.sup.2 under a zero external magnetic field and that of about 10,000 A/cm.sup.2 at the maximum under a magnetic field of 1 T (tesla), which are insufficient for wide application to a cable or a magnet.
Although a solution coating method is also studied for preparation of a wire, this method is extremely inferior to the silver sheath method in view of the critical current density.
In relation to study of a thin film, on the other hand, a thin film having a critical current density of at least 4 million A/cm.sup.2 has been obtained. However, a thin film of such a high property can be formed only on a single crystal of an oxide such as strontium titanate. It is technically difficult to prepare a large oxide single crystal, and such a material cannot be bent for lack of flexibility. Therefore, it is impossible to prepare a long material which can provide a wire, so far as an oxide single crystal is employed as a substrate.